Thermowell - FEA
Thermowell FEA Analysis
A thermowell is a device to protect a thermometer for measuring temperature of an internal flow in a pipeline.
The thermowell is used in almost all process industries where the temperature measurement is involved.
Although the thermowell is a protective device, it is prone to be broken under harsh environment with high temperature,
high pressure and high flow speed. Thermowell breakage can lead to severe damage to the whole engineering system.
Even though, the thermowell can break by different reason, the main cause of the thermowell breakage is due to cyclic loading
on the thermowell which was forced by vortex shedding downstream of the thermowell.
When a fluid flows around a blunt object in its path, vortices are formed downstream of the object.
This is commonly referred to as vortex shedding, Von Karman Vortex Street, or flow vortices.
The Vortex-induced vibration can be observed if the shedding frequency of an object agrees with the natural frequency of the object.
Karman vortex streets generates lateral forces on the thermowell, the thermowell vibrates with the same frequency as the shedding frequency.
If the shedding frequency is correspondent with the natural frequency of the thermowell, the amplitude of thermowell
vibration increases indefinitely, the tip displacement and stresses are greatly magnified, which leads to thermowell breakage.
Von Karman Vortex Street are low pressure cells that are created and shed downstream in an alternating pattern.
The differential pressure due to the alternating vortices produces alternating forces on the object.
This results in alternating stresses on the object as it deflects. While vortex shedding is useful for process flow measurements,
thermowell designer should avoid it due to the potential for failure. So, in addition to process conditions such as pressure, temperature,
and corrosion, the designer must account for the high cycle fatigue strength for overall suitability in the application.
ASME PTC 19.3
The American Society of Mechanical Engineers (ASME) wrote a standard that establishes the practical design considerations for
thermowell installations in power and process piping, known as ASME PTC 19.3 TW-2016.
This standard helps manufacturers and users calculate the suitability of the thermowell in their given process conditions.
This is done by subjecting the designed thermowell to multiple calculations, but the calculation for stresses and vibrations are calculated based on few assumptions
There are 4 quantitative criteria in ASME PTC 19.3 TW-2010 for a thermowell to be found acceptable for a particular set of process condition
Frequency Limit: the resonant frequency of the thermowell must be sufficiently high so that destructive oscillations are not excited by the fluid flow.
Dynamic Stress Limit: the maximum primary dynamic stress must not exceed the allowable fatigue stress limit. If the design requires that the thermowell pass through the in-line resonance to get to the operating conditions, there is an additional fatigue check at resonance.
Static Stress Limit: the maximum steady-state stress on the thermowell must not exceed the allowable stress, as determined by the Von Mises criteria.
Hydrostatic Pressure Limit: the external pressure must not exceed the pressure ratings of the thermowell tip, shank, and flange (or threads).
Most thermowell installations are partially submerged condition and the estimate of actual pressure distribution area on he thermowell is very difficult and the stress calculation on tapered and stepped thermowell shall be a fallacious process.
Also, the mounting assumptions may deviate the results on large extend.
Computational Fluid Dynamics (CFD) Analysis
Computational Fluid Dynamics (CFD) is the analysis of fluid flows using numerical solution methods. The purpose of the CFD simulation is to determine the quantitave and qualitative stress distribution on the thermowell due to the fluid flow.
The pressure distribution and the velocity impact on the thermowell shall be exactly arrived in CFD than the PTC calculation. Also, the CFD to FEA application shall provide more accurate inputs to calculate the stresses on each element of thermowell.
For the tapered thermowells, the FE approach correlates much better than the PTC calculation with the test data. The PTC calculation tends to significantly underestimate the natural frequencies of tapered thermowells.
The FE technique provide a significantly better estimate of the natural frequency of a given thermowell than does the PTC calculation procedure. This is especially true for the tapered thermowell.
Flanged connection to thin walled pipe, FE analysis or physical testing in place may be the best way to obtain an accurate estimate of the true natural frequency.
Steady State and Dynamic Stress Analysis
The steady state stress is a combination of the external pressure from the process as well as the drag force. And the dynamic stresses are the results of periodic drag forces that causes the thermowell to oscillate in the direction of stream and periodic lift forces causes it to oscillate in the transverse direction.
PTC calculates the stresses only at the support plane and the tip, but FEA provides the stress distribution throughout the thermowell and the judgement o dimensions can be done more effectively.